Conveyor belts are used to transport materials such as aggregates, sand and rock by means of a typically rubber, flat belt from a loading end to an unloading end. The conveyor belts may transport the materials horizontally or may be inclined. A well-known use for conveyor belts is to move the material to an elevated discharge where the materials drop into a pile for storage.
Conventionally, a motor to move the conveyor belt drives a head pulley at the top, unloading end of the conveyor. A snub pulley may be provided in conjunction with the head pulley and can increase belt traction by about 20%. Such snub pulleys are conventionally used in conveyor belt systems with high capacities, for example, 400 tons per hour and over. With the drive mounted at the top end of the conveyor, the conveyor belt with the transported material on it is pulled by the head pulley and the belt will tension itself thereby giving good traction on the driving head pulley. In contrast, if the drive is mounted in a return pass of the belt as, for example, at the tail pulley, then the driving tail pulley will have to push the belt. The disadvantage arises that the belt may slacken and slip on the driving tail pulley.
Belt conveyor systems are known in which the drive pulley cannot conveniently be located as the head pulley at the top of a conveyor. One well-known such system is a portable conveying system known as portable radial stackers which are to be transported on the highways. Such stackers typically comprise a conveyor system with three sections, namely, a centre section plus head and tail sections which may be folded up over the centre section to permit the portable stacker to be folded up and travel on a highway within legal transport limits. Such a portable radial stacker is disclosed in CP 1,046,969 issued Jan. 23, 1979. In such portable radial stackers the drive mechanism is advantageously mounted in the centre section to assist in reducing the height of the stacker when folded and to reduce the weight of the head section thus decreasing the size and strength of mechanisms to pivot and support the head section.
It is known in such portable radial stackers to use a dual drive system in which two drive pulleys are provided in a centre section close to each other with the belt passing about one drive pulley and then about the other so as to describe a substantially S-shape in side view.
Dual drive systems are also useful in stationary conveyors and stationary stockpiling stackers. Utilizing a dual drive system somewhere in the centre of a stacker reduces the weight or cantilever load at the head section and, where typical cable suspension support systems are used, helps to reduce winch size and the size of wire rope support cables. Reduced weight also helps to reduce side swaying of the conveyor head section, especially on long, tall cable suspended stackers. The dual drive system is advantageous for any stationary or portable conveyor to be used in limited spaces, for example, in buildings with low clearance. The dual drive system can replace a conventional head pulley drive system and reduce the height of the conveyor in use. Location of the dual drive system in the centre section of the conveyor provides easier access for maintenance and servicing.
In a typical configuration for a dual drive system, a drive motor turns one of the drive pulleys which includes a first pneumatic rubber drive wheel to engage a second pneumatic rubber drive wheel coupled to the other of the drive pulleys. Rotational forces from the motor driving the first drive pulley are transferred via the two frictionally engaging pneumatic rubber tires to drive the other drive pulley. Such configurations suffer a number of disadvantages. One disadvantage is that in forcing the two pneumatic rubber tires into each other as is necessary to minimize slippage, the rubber faces of the tires flex and fight each other causing uneven and premature tire wear. Another disadvantage is that damp sand and fine silts do build up or accumulate on the face of the drive pulley which contacts the load carrying surface of the conveyor belt. This increases the diameter of this pulley slightly and therefore disadvantageously changes the speed at which this drive pulley drives the belt to be different than the speed of the other drive pulley. This difference in speed transfers friction to the two pneumatic rubber drive wheels, resulting in different speeds for the drive wheels and premature tire wear.